The growth of cancer cells in general and of breast cancer in particular depends, in many cases, upon small proteins termed growth factors that will bind and then activate their growth factor receptors. One of these growth factor receptors is the erbB-2 receptor which plays an important role in the prognosis of breast cancer and is expressed at very high levels in nearly 30% of human breast cancer patients. Our recent discovery of gp30/heregulin/NDF has allow us to identify a number of related but distinct biological endpoints which appear responsive to signal transduction through the erbB-2/4 receptor. These endpoints of growth, invasiveness, and differentiation have clear implications for the emergence, maintenance and\or control of malignancy, and represent established endpoints in the assessment of malignant progression in human breast cancer. Preliminary studies in vitro have shown that gp3O induces a biphasic growth effect on cells with erbB-2 over-expression. Strikingly, we have recently observed that the erbB-2 signalling pathway can be modulated by estrogen acting through the estrogen receptor (ER). Conversely, we observed that down regulation of erbB-2 by estrogen can be blocked by gp3O acting through the erbB-2 receptor. Clearly, mechanistic aspects of the erbB-2/4/gp3O interaction need to be understood from a therapeutic standpoint, and may furthermore provide additional insights into treatment synergy for certain patients, or enhance treatment regimens for a large number of women. Understanding the mechanism(s) through which these interactions occur will provide a rational framework with which we examine the therapeutic relevance of the interaction, and may serve to expose other therapeutic targets. These mechanisms will be examined initially using in vitro cell culture and biochemical systems. Ultimately, we hope that these experiments will facilitate the emergence of erbB-2-targeted therapy. We have recently determined the protein sequence of gp3O and obtained its full length cDNA sequence. In addition, we have cloned two additional forms that are believed to be alternatively spliced molecules. We are currently expressing these different forms, in order to determine their biological effects. The availability of gp3O full length cDNA will provide the tools we need to acquire a better understanding of the mechanism of action of the erbB-2 oncogene product and the significance of erbBA in breast cancer. The proposed studies are designed to define the relevance of gp3O in breast cancer tumor progression. Specifically, we will: l) Study the regulation of gp3O by hormones, anti-hormones and differentiation factors, 2) determine if constitutive activation of erbB-2/4 by gp30/heregulin can bypass the normal estrogen requirement of estrogen responsive breast cancer cells, 3) define the mechanism by which gp3O induces breast cancer cells to became hormone independent and to the mechanism of acquiring an hormone resistant phenotype, 4) determine if disruption of the erbB-2/4 activation can restore hormone dependence, and 5) identify isolate and clone genes that are associated with gp3O induction of a more aggressive phenotype. We believe that results from these in vitro and in vivo studies may provide new insights into breast cancer diagnosis, prognosis and treatment.